The hydrogen economy revisited

Hydrogen is not a primary energy source. It does not occur
naturally on earth and can only be extracted from water or other
substances by the application of large amounts of energy, some of
which can be used to perform useful work when the hydrogen is
oxidised to form water. Viewed in this light, hydrogen is just a
way to store large amounts of energy in a transportable form.
Hydrogen can be generated by using any convenient form of energy
to extract it from the source materials. If renewable energy is
used it is carbon neutral. It can, of course, be generated using
nuclear energy from fission, but this seems unlikely when you
consider how little uranium is available (see Fission power). Introducing a
fission driven hydrogen economy would certainly be dependent on
large-scale use of fast-breeder reactors.

The 'traditional' hydrogen economy

Renewable hydrogen

You generate hydrogen renewably by splitting water into
hydrogen gas and oxygen, usually by electrolysis, though biogenic hydrogen may be a
possibility. The oxygen is released into the atmosphere while the
hydrogen is cooled and compressed until it liquifies. The liquid
hydrogen would be taken to where it is needed and used to fly a
plane or fuel a car. Here it releases energy by combining with
oxygen to regenerate the water that was split to generate it in
the first place. Electrolysing water to obtain hydrogen requires
a great deal of energy: you put as least as much energy into
electrolysing water as you get back later from a fuel cell or by
burning it. Liquifying hydrogen also requires a lot of energy:
possibly as much as was used in the electrolysis process. I
haven't seen any firm figures on the overall efficiency of
hydrogen use, so here's a best guess:

The efficiency of direct photovoltaic (PV) generation
varies wildly, from the very expensive space-rated panels on
the ISS (46%) to commercially available silicon PV cells (16%
maximum, probably 9-10% in actual use). If we're talking about
solar power the conversion efficiency doesn't affect this
argument though is does affect the cost since these cells are
not cheap. It just lets us calculate whether we have enough
unused land area to install sufficient generation plant to make
the Hydrogen Economy possible at all. I have no idea of the
efficiency of other forms of renewable electricity generation
so I've omitted them from this calculation.

Assume electrolysis is 80% efficient: little heat is
generated during the process. Compressing the gas for storage
is 90% efficient. If we assume 80% efficiency for using the gas
directly in a heating system or fuel cell the overall
efficiency is 58%: this is a bit better than a CCGT natural gas
generator set. A private communication confirms this estimate:
I'm told that the energy storage system proposed for an
eternal solar-powered aeroplane: electrolysing water to
generate hydrogen, storing the gas and then using a fuel cell
to produce electricity from it has an efficiency of 65%.

Liquifying the hydrogen uses 40% of the hydrogen's energy
content, so this drops the overall efficiency to around 35%.
Thats about the same as you get from using fossil fuel
generated electricity. On efficiency grounds alone, then,
there's probably little between difference between current
energy systems and the Hydrogen Economy

Another contender, direct conversion of solar energy to
hydrogen, emerged in 2012. This process uses a thin (20-30
micrometer) layer of ferric oxide (rust) which is deposited on a
transparent insulating substrate so the ferric oxide forms a
fractal structure with a very high surface to volume ratio. This
is critical: the thickness of the layer needs to be at least 30
uM or the solar absorption efficiency is too low (under 18%),
though this can be improved if the substrate is grooved so that
unabsorbed photons are reflected or refracted back onto the rust
layer. Up to 72% absorption is claimed for this technique. The
surface to volume ratio within the rust layer must also be as
high as possible so that the electrons liberated when ferric
oxide absorbs solar photons can escape to split water molecules
rather than being reabsorbed by the ferric oxide: this limits the
thickness of the rust layer to a few tens of micrometers. Ferric
oxide is inherently more efficient at splitting water than
silicon: the electrons need to carry energy equivalent to 1.23eV
to split water. Silicon emits electrons of 1.11eV or higher
energy while ferric oxide emits electrons carrying 2.1eV or more.
This means that all electrons emitted by ferric oxide have the
energy to split water molecules, which is not the case for
silicon. The end result is that lab systems have already shown
energy conversion efficiencies of 4-5% with the prospect of
matching the best silicon PV systems generating hydrogen by
electrolysis (10%). If this can be achieved, the ferric oxide
process will win on cost because ferric oxide is much cheaper to
make than silicon PV cells and needs no toxic reagents to make
it. The remaining problem is separating the oxygen and hydrogen:
while electrolysis provides physically separated sources of these
gasses the ferric oxide process does not. However, it seem that,
unlike electrolysis, the ferric oxide process will work with
low-quality waste water that contains organic materials and that
these will be preferentially oxidised, thus capturing the oxygen
and allowing relatively pure hydrogen to be collected. The
research on using ferric oxide for solar hydrogen production is
described in New
Scientist, 26 January 2013 page 34.

Current hydrogen sources

However, at present hydrogen isn't produced renewably: it is
at least as big an environmental nightmare as fossil fuel because
it is manufactured by splitting the methane in natural gas to
release hydrogen. This process not only requires a substantial
energy input, probably produced by burning still more fossil
fuel, but releases a carbon-containing exhaust stream, almost
certainly in the form of carbon monoxide and dioxide.

Don't expect hydrogen to appear soon at a gas pump near you,
ready and waiting to be poured into your shiny new eco-friendly
hydrogen fuelled car. The technology is not ready yet. This is
particularly true of the high output fuel cells needed to replace
internal combustion engines. Despite the hype, large fuel cells
have not advanced significantly past the 40 kW produced by the
units in the Apollo spacecraft. Thats 53 horsepower in old money,
or about 43 hp at the wheels of an electric car. So, if the
Hydrogen Economy does appear, expect to drive a modified fossil
fuel engine long before you have an electric vehicle with a fuel
cell. Even if the technology was ready to be rolled out,
economists consider that at least another 10 years would be
required to build enough non-fossil hydrogen plants to meet
demand and to design and install a hydrogen distribution system.
Of top of that there are a few inherent disadvantages that no
amount of technology can overcome:

Liquid hydrogen has a very low density. Despite the fact
that it releases three times the energy of an equal weight of
liquid hydrocarbon, its density is so low that its volumetric
energy capacity is about 25% of the same volume of liquid
hydrocarbon, requiring the fuel tank to be four times the size
of a liquid fossil fuel tank for the same range per fill.
Consequently, the size of liquid hydrogen tanks presents major
design and engineering problems for road vehicles and
aircraft.

There is a problem with keeping the hydrogen cold and
liquified during transport or in a hydrogen powered vehicle's
tanks. Currently these tanks can only keep the hydrogen liquid
by venting hydrogen gas. This cools the tank by evaporating
some of the gas. Evaporation requires energy, so the remaining
liquid is cooled by extracting the latent heat of evaporation
from it. The result is dire: BMW's experimental Hydrogen
Seven looses much of its boot space to the "barrel sized"
cryotank, which still only holds enough for somewhat over a 100
mile range. What's worse is that the cooling system empties the
tank in nine days even if the car isn't being driven. This
means that a fleet of cars with cryogenic tanks could easily
"leak" most of the hydrogen put into their tanks, potentially
harming the atmosphere, but also multiplying the cost and
pollution of hydrogen production by quite a large factor. The
BMW uses 11% of the tank's capacity per day for cooling, so if
its driven 100 miles per day and then refuelled its energy
efficiency is reduced by another 10%. This makes the overall
fuel efficiency 32% at best.

You can't park a car with cryogenic tanks in a basement,
garage or other enclosed space due to the risk of fire from the
evaporating hydrogen, which collects in the parking area.

Smoking might be a remarkably bad idea in a street where
hydrogen fuelled rush-hour traffic is stalled at a congested
intersection.

Hydrogen leakage during handling is estimated at about 15%.
Add in the 10% lost per day to cryogenic cooling and leakage is
now 25%. In a Hydrogen Economy megatonnes of it would be
produced annually and 250,000 tonnes would be released into the
air for every megatonne produced. Nobody knows what effect this
might have on the atmosphere.

Vehicle crashes would probably leak fuel more often, though
this may not be as bad as it sounds. The gas naturally floats
up and away, unlike petrol vapour, which is heavy enough to
flow along the ground and find a spark or a cigarette. Although
hydrogen is more flamable than petrol its colourless flame
radiates less heat. Finally, the burning gas floats up and away
instead of running along the ground incinerating anything in
its path. All in all this is probably an even call.

You get a lot of water produced when you burn hydrogen
or run a fuel cell on it. This table shows the tonnes of carbon
dioxide (CO2) and water
(H2O) produced when one tonne
of hydrogen or jet fuel is burnt:

Fuel

Composition

CO2

H2O

Hydrogen

H2

0

9.00

Jet Fuel A

C12H26

3.11

1.38

Hydrogen produces three times as much energy per tonne as Jet
Fuel A when its used in an engine or fuel cell (New Scientist, 9 July 2005 page
23). After you factor that in, that hydrogen-powered jet flight
will still dump 2.2 times as much water in the atmosphere as the
same flight powered with fossil fuel. The relative water
production figures for petrol and oil versus hydrogen are
unlikely to differ much from jet fuel, so the same calculation
applies to hydrogen powered road vehicles. While water in the
exhausts of hydrogen powered vehicles is just replacing the
amount that was split to obtain the hydrogen, its now in a very
different place. This has knock-on effects:

Recall the impact on the climate of jet contrails. In the
three days after 9/11, when all civilian flying was banned, the
surface temperature in the USA rose 1°C thanks to the
absence of altocirrus caused by jet contrails. Now consider the
effect of burning something that releases over twice as much
water per flight.

The summer microclimate in big cities is hot and humid
enough already. Now double the contribution to humidity from
vehicle exhausts on a summer day. Welcome to the sweatbox.

In 2005 aviation contributed 3% of the human-generated carbon
dioxide input to the atmosphere. This is projected to rise to 6%
by 2050. Commercial jet travel's input is thus just a small part
of the human-generated green-house effect. However, water vapour
from jet travel has already been shown to have a direct,
deleterious and measurable effect on the weather. I think on
balance I'd prefer to stick to fossil fuel for civil aviation if
the only alternative is liquid hydrogen.

However, don't just take my word for this. Robert Zubrin, who
is an aerospace engineer, is even more scathing about the
hydrogen economy than I am. The
Hydrogen Hoax is his analysis. He is president of the R&D
form Pioneer
Astronautics.

A "hydrogen on demand" system, also known as a hydrogen
battery, generates the hydrogen as it is needed at the place
where it will be used, extracting the energy for this from a
precursor substance. The resulting hydrogen is then used to run
an engine or cause a fuel cell to generate electricity. Such a
system has major advantages over the 'traditional' hydrogen
economy:

There is no need for bulk hydrogen storage

The hydrogen is never stored nor transported so leakage is
minimal and fire risk is reduced

Hydrogen gas is light and bulky, so must be liquified for bulk
storage and transport. Liquifying it uses 40% of the energy
content of the stored hydrogen, so almost anything that can avoid
this requirment is worthwhile. Storage tanks for liquid hydrogen
are heavy, bulky, and consume energy running the cryogenic
systems that are needed to keep the hydrogen in a liquid
form.

Possible "Hydrogen on demand" systems

These differ in precursor material and hence in the chemistry
involved.

Aluminium

Engineuity is
working on a system using the reaction between aluminium and
water to liberate hydrogen. The end of a roll of aluminium wire
is lit and dipped into water. The resulting mixture of steam and
hydrogen is burnt in a conventional engine. The system may also
use magnesium or boron instead of aluminium. The metal oxide
would be returned to a reprocessing plant where the metal wire
would be regenerated via electrolysis. No efficiency estimates
are available. The system is claimed to be a zero emission
technology with the possibility of eliminating nitrogen oxides
completely if the water is replaced with a solution of hydrogen
peroxide.

The company expects to have a prototype running in 2009.

Boron

This system has been proposed by a team from the University of
Minnesota and Weizmann Institute at Rehovot led by
Tareq Abu-Hamed. Unlike some of the other schemes, the
complete fuel cycle's efficiency has been evaluated and it has
been designed from the outset as a zero emission system.

Pure powdered boron is reacted with water vapour at 800°C.
The boron combines with the oxygen in the water molecules to form
boron oxide and hydrogen gas. The hydrogen is used in a
conventional engine or fuel cell to produce energy and water,
which can be recycled into the boron reactor. The boron oxide is
periodically swapped for a fresh supply of boron. The oxide is
returned to a solar-powered plant that reduces it to pure boron.
This is achieved by reacting it with magnesium. The resulting
magnesium oxide is in turn reacted with chlorine, which forms
magnesium chloride and returns oxygen to the air. Solar energy is
used to melt and electrolyse the magnesium chloride at 700°C.
Both the magnesium and the chlorine are continuously recycled
within the plant. The result is zero pollution power because the
engine emits only water and the boron reduction process emits
only oxygen.

The substances needed to power a vehicle, boron and water, are
lighter, safer, and easier to handle than hydrogen in high
pressure cylinders. Boron, a black powder is highly flammable. It
does not spontaneously combust or react with liquid water at
normal temparatures, so it may be safer to store and transport
than petrol. It is three times denser than petrol, so occupies
proportionately less space.

27 kg of boron and 68 kg of water are needed to produce 7.5 kg
of hydrogen, which contains the same energy as 60 litres (48 kg)
of petrol and would occupy 30% more space. On the assumption that
the solar cells driving the electrolysis process is 35%, the
overall efficiency of this system is 11% - about the same as a
current petrol engine.

Magnesium hydride

Safe
Hydrogen is developing a hydrogen storage and generation
technology using magnesium hydride slurry as a pumpable hydrogen
fuel. Hydrogen is generated as needed by mixing the slurry with
water in a special mixing device. A pilot project used lithium
hydride slurry to fuel a converted internal combustion engine in
a Ford truck.

The hydrogen density of magnesium slurry is twice that of
liquid hydrogen. It is claimed to be safe to transport and store
at normal temperature and pressure. The slurry can be distributed
using the existing fossil fuel infrastructure of tanks, trucks
and pipelines. Depleted slurry can be completely recycled into
fresh hydrogen-dense slurry. The overall thermal efficiency is
said to be similar to liquid hydrogen, i.e. somewhere between 8%
and 18% depending on the source of the hydrogen.

Sodium boro-hydride

Daimler-Chrysler's Natrium
project demonstrated a car using sodium borohydride as the
precursor substance. The vehicle was a standard Town &
Country petrol minivan fitted with the replacement drive
train.

When a 25% aqueous solution of sodium borohydride is passed
over a catalyst of the precious metal ruthenium, it reacts with
water to produce hydrogen and a solution of sodium metaborate.
The hydrogen was used in a fuel cell to generate electricity
which drove a 35 kW electric motor and recharged a 40 kWh
lithium-ion battery, which was also charged from regenerative
braking. The hydrogen generator and fuel cell were supplied by
Millennium Cell, which went into voluntary liquidation in
October, 2008. The car had a top speed of 130 kph and a range of
500 km. The drive train is heavy: the Natrium was about 226 kg
heavier than the original petrol car.

The cost, in both energy and financial terms of reprocessing
sodium metaborate to regenerate the sodium borohydride is
currently high. In 2003, sodium borohydride cost about 50 times
as much as the energy-equivalent amount of petrol. The current
Brown-Schlesinger process is 50 years old and is chemically quite
inelegant, using methane, processed boric acid and metallic
sodium as raw materials. The main source, Rohm and Haas's specialty chemical
division only produced about 15 tons a year. In 2005
Millennium Cell claimed to have made a breakthrough in
producing sodium borohydride but details were not available. They
were predicting a 4 fold drop in cost, but while this might have
made their technology suitable for military use, it would have
been too expensive to replace fossil fuel.

Despite showing respectable range and performance, the
Natrium project was terminated in 2003 because of the
difficulties of providing the fuel transport infrastructure and
making it efficient and environmentally friendly. The 25% sodium
borohydride solution is non-flammable and non-explosive and the
waste sodium metaborate solution is safe to handle.